Bonding between boron atoms is more complex than in carbon; for example, both two- and three-center B—B bonds can form. The interaction between these bonding configurations results in as many as 16 bulk allotropes of boron, composed of icosahedral B12 units, small interstitial clusters, and fused supericosahedra. In contrast, small (n<15) boron clusters form simple covalent, quasi-planar molecules with carbon-like aromatic or anti-aromatic electronic structure. Recently, it was shown that B40 clusters form a cage-like fullerene, further extending the parallels between boron and carbon cluster chemistry.
To date, experimental investigations of nanostructured boron allotropes are notably sparse, partly due to the costly and toxic precursors (e.g., diborane) typically used. However, numerous theoretical studies have examined 2D boron sheets (i.e., borophene). Although these studies propose various structures, the general class of 2D boron sheets is referred to as borophene. Based upon a quasi-planar B7 cluster (FIG. 1A), an Aufbau principle has been proposed to construct nanostructures including puckered monolayer sheets (analogous to the relation between graphene and the aromatic ring). The stability of these sheets is enhanced by vacancy superstructures or out-of-plane distortions. Early reports of multiwall boron nanotubes suggested a layered structure, but their atomic-scale structure remains unresolved. It is therefore unknown whether borophene is experimentally stable and whether the borophene structure would reflect the simplicity of planar boron clusters or the complexity of bulk boron phases.